1. Field of the Invention
The present invention relates to a heating apparatus used for an image forming apparatus such as a copier, a laser beam printer, or a facsimile, and to an image forming apparatus including the heating apparatus.
2. Description of the Related Art
A heating fixing apparatus, which is included in an image forming apparatus such as an electrophotographic copier or laser beam printer, fixes an unfixed image (toner image) formed on a transfer sheet by an image forming method such as an electrophotographic process onto the transfer sheet. As an example of the heating fixing apparatus, Japanese Patent Application Laid-Open No. 2009-186933 discloses a heating-roller-type heating fixing apparatus including a halogen heater as a heat source and a film-heating-type heating fixing apparatus including a ceramic heater as a heat source.
In general, a heater as a heat source is connected to a commercial AC power supply via a switching element such as a bidirectional thyristor (hereinafter referred to as a “triac”) and is supplied with power from the commercial AC power supply. The heating fixing apparatus including the heater as a heat source is equipped with a temperature detection element, for example, a thermistor thermosensitive element. Further, the temperature detection element detects the temperature of the heating fixing apparatus, and a sequence controller performs on/off control of the switching element based on the detected temperature information. Thus, power supply to the heater as a heat source of the heating fixing apparatus is performed or stopped, and hence temperature control is performed so that the temperature of the heating fixing apparatus becomes a target temperature. The on/off control for the ceramic heater is performed usually by phase control or wave number control of the commercial AC power supply.
In order to adjust the temperature of the heating fixing apparatus, the sequence controller compares the temperature detected by the temperature detection element with a predetermined target temperature so as to calculate a ratio of power to be supplied to the heater. Further, the sequence controller determines a phase angle or a wave number corresponding to the calculated power ratio, and performs on/off control of the switching element based on the phase condition or the wave number condition.
Next, a description is given of a switching power supply having a unit for detecting that the commercial AC power supply voltage has dropped. In the switching power supply, when a switching action continues in a state where the commercial AC power supply voltage is dropped, a rated current value may be exceeded due to an overcurrent state of the switching current, or stress may be applied to each element due to abnormal heating of a circuit component. Therefore, Japanese Patent No. 3372914, for example, discloses a switching power supply that monitors a DC voltage value after the AC voltage is rectified and smoothed, in order to detect that an input voltage from the commercial AC power supply has dropped.
FIG. 10 is a functional block diagram of a switching power supply disclosed in Japanese Patent No. 3372914. The switching power supply of FIG. 10 includes a rectification circuit 2 which inputs an AC voltage VAC from an AC power supply 1 and converts the AC voltage VAC into a DC voltage, and a stabilization power circuit 7 which rectifies and smoothes the DC voltage as an input voltage VIN smoothed by a smoothing capacitor 3 so as to output a DC output voltage VOUT. Further, the switching power supply includes an input shut-down detection circuit 5 which monitors the AC voltage VAC and outputs an input shut-down signal when the input is shut down, and a low input voltage protection circuit 6 which monitors the input voltage VIN and stops operation of the stabilization power circuit 7 when the input voltage VIN becomes a predetermined voltage value or lower. The low input voltage protection circuit 6 detects whether or not the voltage value of the DC voltage VIN smoothed by the smoothing capacitor 3 is a predetermined voltage or lower based on dividing resistors in the low input voltage protection circuit 6. Then, when it is detected that the voltage value of the DC voltage VIN is dropped to the predetermined voltage or lower, the low input voltage protection circuit 6 operates to stop the output from the stabilization power circuit 7, and hence components such as elements in the input side as a primary side are protected.
However, in the switching power supply disclosed in Japanese Patent No. 3372914, when the input voltage from the commercial AC power supply gradually drops, and the low input voltage protection circuit 6 operates, the following problem occurs. Specifically, in the case where record data is being written in a recording device such as a hard disk drive by using the DC output voltage VOUT of the switching power supply, if the low input voltage protection circuit 6 operates to stop the output of the switching power supply, the power supply to the recording device is shut down. As a result, the record data that is being written in the recording device may not be written in a normal way, and hence the record data may be damaged.
Therefore, in order to solve this problem, it is necessary to safely stop the writing operation of the recording device before the output of the switching power supply is stopped. In the circuit structure of Japanese Patent No. 3372914 illustrated in FIG. 10, when the low input voltage protection circuit 6 detects that the input voltage VIN is a predetermined voltage or lower, the recording device is notified that the output of the power supply will be stopped in a predetermined time, and hence the recording device stops the writing operation. Then, after the predetermined time passes, the output of the stabilization power circuit 7 is stopped so that a damage to the record data can be avoided. In addition, there is also a method of providing the switching power supply with a low voltage detection circuit separately, the low voltage detection circuit performing low voltage detection of the input voltage from the commercial AC power supply at a voltage higher than the detection voltage of the low input voltage protection circuit 6. By this method too, it is possible to detect that the input voltage from the commercial AC power supply drops, and to stop the writing operation of the recording device before stopping the output of the stabilization power circuit 7.
However, in order to realize this method, it is necessary to dispose an additional delay circuit for stopping the output of the stabilization power circuit at the timing after a predetermined time, or an additional low voltage detection circuit for detecting that the input voltage from the commercial AC power supply drops. Therefore, there is a problem that the circuit scale and cost are increased.